1. Field of the Invention
The present invention relates to an optical disk and the like set in an optical disk unit, and a method for recording, reproducing, and deleting information on and from such an optical disk.
2. Description of the Related Art
In recent years, rewritable optical disks have been studied and developed. As a result, some rewritable optical disks have already been put into practical use as external memories for computers. Such conventional optical disks have spiral guide grooves formed thereon, and groups of groove-shaped pits are formed on spaces between the adjacent guide grooves (hereinafter, such spaces are called lands). Information is recorded on, or reproduced or deleted from the optical disk by focusing a light beam on the land between the guide grooves. Positional information indicating the position on the optical disk is produced by the arrangement of each group of pits, so that the light beam can be guided to a target position on the optical disk.
In order to increase the recording density of the optical disk, Japanese Patent Publication No. 4-27610 proposes to record information on both the guide grooves and the lands, both of which have widths that are made substantially the same. In this proposed method, however, since only one light beam is used for the guide grooves and the lands to effect the recording, reproduction, and deletion of information, the data transfer rate will not be improved compared with the preceding optical disks. Further, the above prior art does not describe how the light beam accesses a target position except for the mention of using an address signal.
In order to improve the data transfer rate, a multibeam method where information is recorded, reproduced, and deleted by using a plurality of light beams has been vigorously studied. Japanese Laid-Open Patent Publication No. 3-214428, for example, proposes to provide one unit of address information for a plurality of tracks.
FIG. 4 of "Development of High Speed Magneto-optical Disk Drive Using 4 Beam Optical Head" (Paper of Electronic Information Communications Society, Sep. 25, 1990, MR90 31, pp. 37-44) discloses the structure of an optical disk where information is recorded, reproduced, and deleted by using a plurality of light beams. According to this disclosure, a guide groove at a pitch of 6 .mu.m and groups of pits (Preformat IDs) arranged parallel to the guide groove are formed on the optical disk. An array of four light beams are positioned inclined with regard to the guide groove. One of the four light beams moves on the guide groove irradiating the guide groove. Another light beam adjacent to the one on the guide groove reproduces information stored by the pits (hereinafter, such information is referred to as pit information).
According to the above conventional optical disk, one of the four light beams is used to record information on the guide groove while the other three are used to record information on flat lands of the optical disk. Since the thermal diffusion at the guide groove is different from that at the land, the recording power of the respective light beams is required to be individually optimized to obtain a uniform recording status. Further, when the inclination of the array of light beams with regard to the guide groove changes, each of the light beams, other than the one located on the guide groove, moves on a line deviated from the line where information has been recorded.
In order to solve the above problem, "Feasibility Study on High Data Transfer Rate of 300 Mbit/s with 8-beam Laser Diode Array" (ISOM/ODS'93 Conference Digest, pp. 53-54) discloses a method for adjusting the inclination of eight light beams with regard to guide grooves, while using a preceding optical disk, so as to precisely position the eight light beams on eight lands between the guide grooves.
In the above multibeam method disclosed in Japanese Laid-Open Patent Publication No. 3-214428, as shown in FIG. 2 thereof, groups of pits for producing address information are formed in line with guide grooves, not on lands where information is recorded, reproduced, and deleted. With this structure, each of a plurality of light beams is required to be split into three light beams so as to allocate one of the split three light beams for reading pit information produced by the pits formed in line with the guide grooves. This results in complicating the structure of an optical head. Also, since each of the plurality of light beams for recording and deleting information is split into three, the intensity of the main light beam allocated for recording and deleting information is lowered. This necessitates to use a semiconductor laser with a higher output as a light source. Moreover, since only the lands are used for the recording, reproduction, and deletion of information, the recording density is as low as that of the preceding optical disks.